In gate turn-off thyristors and transistors of large capacity a high current flows through the control electrode (the gate or the base) in accordance with an increase in capacity; in recent models in commerce tens to a few hundred amperes of current flows. Particularly, in gate turn-off thyristors of large capacity a large gate reverse current is required at the time of turn-off. In addition, it is essential to ensure that the current flows evenly and instantaneously. Therefore it has been proposed that the impedance between the gate and the cathode be minimized. For example, the shape of the access portion for the gate is shaped like a ring, thereby shortening the distance from the access portion up to the emitter region, equalizing the current distribution, and reducing the current.
To explain the background of the invention, reference will be more particularly made to FIG. 1, which shows an example of a conventional semiconductor device. FIG. 1 illustrates a pattern in a plane view of a gate turn-off thyristor element having a ring-shaped gate structure, and FIG. 2 shows a cross-section thereof. The reference numerals 1, 1a, 1b, 1c, and 1d designate a thyristor element, a silicon wafer, a reinforcing plate of molybdenum having the wafer 1a soldered thereto (the pewter being designated by 1g), a gate electrode, and a cathode electrode, respectively. Both of the reference numerals 1e and 1f designate current collecting portions of the gate.
In this example the gate electrode 1c and an external gate electrode (not shown) are directly interconnected to each other through wiring. FIG. 3 is a perspective view showing an access structure for the gate electrode, and FIG. 4 is a cross-section thereof. The reference numeral 2 designates an aluminum wire, which normally has a diameter of 0.3 to 0.7 mm. It is necessary to fix or bond the aluminum wire 2 at several points along the ring-shaped current collecting portion of the gate 1e so as to equalize the distribution of the gate current, wherein the intervals therebetween are preferably equal.
In addition, in such large-capacity gate turn-off thyristors the common practice is to insert a plate between the cathode electrode 1d and an external cathode (not shown) so as to minimize heat stress. The plate is made of molybdenum, tungsten or the like, which has approximately the same thermal expansivity coefficient as that of silicon. It is also essential to fix the plate at an exact position with respect to the cathode electrode 1d. To this end it is necessary to adhere the plate on the cathode electrode 1d of the thyristor element 1 with a special adhesive, such as a heat-proof high molecular substance. As a result, under the conventional practice the assembly process requires much manual labor, which unfavorably reflects in the production efficiency and the price. In addition, the wire bonding portion is likely to break or to be damaged due to mechanical stress or heat stress occurring in the heating cycle.
Furthermore, under the conventional method the electrical resistance of the wire itself must be taken into consideration, otherwise it would prevent discharging of carriers from the gate when the main current is cut off, particularly when a few hundred amperes of current flows. This often leads to reduction of the controllable current of the gate turn-off thyristor.
Another example of the conventional gate electrode structure employing the same wire bonding as that in the above-mentioned example is disclosed in Japanese Patent Laid-Open (Kokai) No. 53-95583. This gate turn-off thyristor has a gate lead directly bonded to both the semiconductor element and the external electrode. This prior art has the same disadvantages as those of the above-mentioned example.
As the capacity of gate turn-off thyristors becomes large, the diameter of the elements increases, and this makes it necessary to lengthen the wirings for bonding. At the same time the gate current required at the time of cut-off also increases, and this makes it necessary to reduce the resistance in the wirings to a greater extent.
In order to supply sufficient current to the gate by reducing the resistance in the wirings, it is the common practice to keep the gate access electrode in contact with the element. This method is effective in a center gate thyristor where the gate is located at the center of the element, and in a large-capacity transistor.
FIG. 5 is a cross-section showing the thyristor constructed so as to keep the gate access electrode in pressure-contact with the thyristor element, which is disclosed in Japanese Patent Laid-Open (Kokai) No. 57-62562. This disclosed semiconductor device includes a thyristor element 1, an inserted plate 3, an outer cathode electrode 4, a gate access lead 5, an insulating support 6 whereby the gate access lead 5 and the inserted plate 3 are exactly positioned with respect to the outer cathode electrode 4, a spring 7 whereby the top portion 5a of the gate access lead 5 is kept in pressure-contact with the thyristor element 1 through the insulating support 6, a protection pipe 8 whereby the gate access lead 5 is insulated from the outer cathode electrode 4, an outer anode electrode 9, a ceramic cylinder 10 for supporting the thyristor element 1, a cathode-side flange 11 for fixing the outer cathode electrode 4 to the ceramic cylinder 10, an anode-side flange 12 for fixing the outer anode electrode 9 to the ceramic cylinder 10, and an outer gate electrode 13.
In this example it is not necessary to bond the gate access electrode and the gate access wire together, which makes it possible to use a wire having a large diameter, and a wire made of a more conductive substance than aluminium, such as silver. This is particularly advantageous for supplying a high current.
However, since the insulating support 6 must be made of sintered alumina or the like, it is difficult to produce it to exact dimensions, which necessitates relatively large clearances between the outer cathode electrode 4 and the insulating support 6, and between the insulating support 6 and the gate access lead 5. This leads to the inexact positioning. Owing to this drawback this gate access structure is not applicable to the gate turn-off thyristors which require highly exact positioning.
There is still a further example for ring-shaped gate turn-off thyristors disclosed in Japanese Patent Laid-Open (Kokai) No. 58-148433. The disclosed prior art adopts the electrode pressure-contact system, characterized in that a gate ring, divided into several sections, is kept in contact with the ring gate. However, in this example the gate lead is supported by an insulating support of ceramic or the like, which leads to the inexact positioning as pointed out with respect to the above-mentioned example.
In the second example mentioned above it is necessary to provide a gate access lead 5 having a ring-shaped-top portion 5a which is to be kept in pressure-contact with the thyristor element 1; in fact, however, pressure-contact type gates securing an easy and exact positioning of the ring-shaped top portion have not materialized.